This invention relates to dispersions of fine porous inorganic oxide particles having a median particle size of three microns or smaller. The invention also relates to methods of making these dispersions.
Fine sized inorganic oxide particles are well known. For example, xe2x80x9ccolloidal silicasxe2x80x9d are well known and typically have median particle sizes (diameters) of 0.1 microns (100 nanometers) or less. See, for example, Canadian Patents 609,186 and 609,190; and U.S. Pat. No. 3,012,973. Colloidal silica in a dispersed, non-aggregated state is not porous, and any surface area present in these particles is on the external surface of the primary, non-aggregated particles, i.e., there is no surface area provided by internal porosity in the particles.
Dispersions of relatively small silica gel particles known as xe2x80x9csilica miocrogelsxe2x80x9d are disclosed in U.S. Pat. No. 4,954,220. Silica microgel particle dispersions are prepared by reacting alkali metal silicates and acid under conditions to initiate the coalescence and gelling of primary particles. The conditions, however, are also selected so that the gelled particles only grow to small, e.g., submicron, sizes. The dispersion in which microgel particles are prepared is not processed to the point that the coalescing particles form a rigid macrogel. Microgels are typically prepared from sol dispersions having less than 10%, and more typically less than one percent by weight solids.
European Patent Application 468,070 discloses preparing a dispersion of gel particles having particle sizes in the range of 1 to 50 microns, preferably in the range of 10-30 microns, by wet milling larger gel particles. The particles resulting from the wet milling are reported to have surface areas ranging from 50 to 700 m2/g and pore volumes of 0.3 to 2 cc per gram. The wet milled particles are then spray dried to make larger spherical particles in the range of 1 to 200 microns, preferably 30-100 microns. This patent discloses using the spray dried particles as carriers for polyolefin catalysts and that the larger spherical particles in the range of 30-100 microns are preferable.
U.S. Pat. No. 5,030,286 discloses high solids content dispersions, e.g., from 40 to about 55% by weight, of precipitated silica. The precipitated silicas illustrating the dispersions of this patent have average particle sizes greater than one micron. The dispersions are prepared by milling the precipitated silica. It is reported in the ""286 patent that milling the precipitated silica results in a less viscous dispersion and allows the dispersions to be used at high solids concentrations. The ""286 patent discloses that these high concentration dispersions can be used in paper coatings.
European Patent Application 803 374 discloses using agglomerated silica particles having average particle sizes of 10-300 nm in ink receptive layers for ink jet paper. It is disclosed that these particles are prepared from grinding larger particles, e.g., having sizes of 9xcexc. PCT Application WO 97/48743, discloses wet milling inorganic oxide gels to obtain average particle sizes of 3 to 16 microns. This patent application discloses that any particles below 1 micron are present at a maximum amount of 40 percent by weight. The particles described in this application are then spray dried to form catalyst supports.
U.S. Pat. No. 4,983,369 discloses a dispersion of dense spherical particles having average particle sizes of 0.1 to 10 microns. The ""369 patent also discloses that the spherical particles can be either nonporous or porous. The ""369 patent also discloses that the porosity of the porous particles is reduced when contacted with water. The porous particles made by the process described in the ""369 patent have particle sizes greater than one micron.
U.S. Pat. No. 5,277,888 discloses using organic dispersing medium to make stable dispersions of particles having average sizes in the range of 0.05 to 10 microns. The patent illustrates the dispersion with dense non-porous silica particles.
U.S. Pat. No. 2,731,326 discloses aggregates of dense, low porosity silica spheroids wherein the aggregates are described as supercolloidal, i.e., larger than 100 millimicrons (0.1 micron). These aggregated particles have primary particle sizes greater than 15 nanometers and are described as having a size such that they appear as a precipitate when they are prepared in dispersion.
U.S. Pat. No. 2,741,600 discloses preparing dispersions of silica having average particle sizes below 1 micron, e.g., of between 0.1 and 0.5 microns. The dispersions disclosed in the ""600 patent are produced by milling autoclaved silica gels. The material resulting from the autoclaving step is described as possessing two phases, i.e., an aquasol phase and a xe2x80x9cmudxe2x80x9d phase. The mud phase is milled to form a dispersion described as having a milky white appearance. The two phases can be used separately, or as a combination. Porosity of the milled mud phase or the aquasol phase produced during autoclaving is not reported nor described in this patent.
Iler""s xe2x80x9cChemistry of Silicaxe2x80x9d, pages 328-330, (1979) discloses porous silica sols which are prepared through various procedures. The porous silica sols described in Iler""s treatise range in size from 0.1 to 0.3 microns. In general, these submicron sized particles are prepared by aggregation of smaller particles. The surface areas of the particles produced are reported to be in the range of 22 m2/g to over 700 m2/g, depending on the methodology used to prepare the particles. For example, U.S. Pat. No. 3,607,774 referred to by Iler discloses silica sols of 0.5 microns or greater, e.g., up to 1.5 microns. The ""774 patent reports making sols having surface areas of about 103 m2/g or less. Another patent referred to by Iler, et al., U.S. Pat. No. 3,591,518 discloses porous silica sols making particle sizes of about 0.2 microns and external surface areas in the range of 17-900 m2/g. Pore sizes, pore volume and pore volume stability for the particles of the two aforementioned patents are not reported.
WO 97/22670 discloses precipitated silicates and slurries prepared by destructing aluminum silicate and silica via dry milling and then formulating slurries by addition of water. Slurries comprising the destructured materials have reduced viscosities compared to slurries of unmilled silica at equal solids content. Those slurries also have reduced DPB values. The median particle size for these particles is significantly greater than one micron.
U.S. Pat. No. 5,318,833 discloses an ethylene glycol based slurry comprising precipitated silica coagulates having an average particle diameter of 0.5 microns. The precipitated silica has a pore volume of 0.09 cc/g and a surface area of 70 m2/g. The ethylene glycol slurry is combined with a phthalate to prepare polyester film. The precipitated silica is used to impart friction properties to the polyester film so the film can be readily processed later, e.g., to make magnetic recording media. These films need to have friction properties such that the film does not adhere to the machinery which handles the film. However, in using additives such as particulated silica, care also has to be taken to not detrimentally affect other properties of the polyester. It is also envisioned that the particles disclosed in this patent do not provide sufficient surface area for bonding to the ethylene glycol. As a result, the chances are greater than the particles and ethylene glycol polymer can separate, thereby causing defects in the resulting film.
As indicated above, a dispersion of particles can also be used to form porous structures, e.g., ink receptive coatings for paper. It is desirable if the porosity in such coatings not only provides a means for increasing the rate of ink absorption for certain paper coatings, but also provides for relativley high ink capacity in order to retain high resolution images. Large sized porous gels, i.e., greater than three microns, have been suggested for this purpose. Submicron colloids have also been used, but the porosity in colloid particles themselves, i.e., internal particle porosity, is not-existent. Therefore any porosity created by colloidal particles is interparticle porosity created after the dispersion of colloid is dried. Interparticle porosity is affected by a number of factors which need to be carefully controlled if relied upon as the sole source of porosity.
Many of the paper-related applications also require fine sized, e.g., submicron, particle sizes. For example, it is known that paper coatings of relatively high gloss can be achieved with dispersions of submicron colloid silica particles. In the event it is desirable for a glossy coating to be ink receptive, it would be a further advantage that the particles in the dispersions be porous. Therefore, there is a continuing need to develop dispersions of fine sized, porous inorganic oxide particles.
It also is desirable to employ fine sized, i.e., particles having an average particle less than one micron, to modify the friction properties of polyester films. However, up until now, such particles have had relatively low pore volume. Without being bound to any particular theory, it is believed that high porosity materials improve the bonding of the organic polymer to the inorganic particulate, and thereby decreases film defects caused when polymer separates from the inorganic particulate.
The dispersion of this invention comprises porous inorganic oxide particles which have a median particle size in the range of 0.05 to about 3 microns. The particles of these dispersions have a porous structure such that at least about 0.5 cc/g of the pore volume is from pores having a pore size of 600 xc3x85 or less. Porosity from pores less than 600 xc3x85 is referred to herein as internal porosity, i.e., porosity present in the particles themselves. Indeed, the internal porosity is reflected by the dispersion having a xe2x80x9cviscosity derived pore volumexe2x80x9d, defined later below, of at least about 0.5 g/cc. The internal porosity of the particles in this invention is relatively stable and reliable. It is believed that the particles of this invention comprises a coherent, relatively rigid three dimensional network of contiguous particles sometimes referred to as xe2x80x9cultimatexe2x80x9d or xe2x80x9cprimaryxe2x80x9d particles. The bond between the primary particles of this invention are strong, therefore resulting in an rigid aggregate particle. The particles therefore do not completely collapse under capillary pressures created when the water evaporates from the dispersion during drying, and porosity within the particle is maintained.
Embodiments of this invention comprising silica gel particles also offer porosity which is maintained throughout the dispersion""s preparation by milling. For example, the pore size distribution of embodiments comprising silica gel dispersions is relatively the same as the distribution in the gel from which the dispersion is prepared. This is because the porosity of the starting material is homogeneous and, in general, below 600 xc3x85. There also are embodiments of gel particles in which at least about 0.7 and 0.9 cc/g of pore volume is from pores having sizes less than 600 xc3x85. In these embodiments, at least 80% of the pore volume is from pores having pore sizes less than 300 xc3x85. Pores of that size are not as affected by milling and the homogeneity of that porosity insures that the porosity is relatively consistent from particle to particle even after being milled to submicron sizes.
The dispersions are prepared by forming a slurry of inorganic oxide particles and milling the slurry under conditions and in a manner sufficient to create a dispersion comprising particles having a median particle size of about three microns or smaller. In embodiments in which it is desired to create stable dispersions xe2x80x9cin situxe2x80x9d without the aid of dispersants, or if it is desired to prepare dispersions having a median particle size less than one micron, the process comprises
(a) forming a slurry of inorganic oxide particles,
(b) milling the slurry,
(c) creating a supernatant phase and a settled phase from the resulting milled slurry; and
(d) removing the supernatant or redispersing the settled phase as a final product, wherein said product has a porosity after being dried in which at least 0.5 cc/g of pore volume is from pores having a pore size of 600 xc3x85 or smaller.
Process step (c) is preferably carried out using a centrifuge, and especially preferred when particles having a median particle size of about one micron or smaller are desired. In general, particles larger than one micron settle out from the dispersion when centrifuged, and the settled phase can be removed to leave a dispersion of particles having a median particle size of one micron or smaller.
The porosity and size of the inorganic oxide particles produced by this embodiment make the particles particularly suitable for use in a glossy ink receptive coating on paper or film. For example, a relatively high gloss appears on coatings containing submicron sized inorganic oxide compared to the gloss seen on coatings containing larger-sized particles (average size greater than 1 micron).
The submicron inorganic oxide particles of this invention also have an improved, reliable capacity for ink absorption compared to traditional, non-porous colloidal inorganic oxides for the same nominal particle size. As mentioned above, the porosity is provided by relatively rigid particles and is less subject to being reduced upon drying. Thus, one can rely on porosity being present after coatings containing the invention are dried.